Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Multiple-input multiple-output (MIMO) communication systems employ multiple transmit antennas and multiple receive antennas to communicate data symbols over a communications channel. MIMO communication systems may allow a plurality of receivers to be serviced utilizing a same frequency band. In this manner, MIMO communication systems may advantageously increase an amount of data the communication systems are able to send to users.
MIMO systems may find use in a variety of applications including, but not limited to, wireless networks, cellular systems including 3G and 4G systems, such as 3GPP LTE-Advanced, local and wide area networks, and wireless broadband systems (such as WiMAX).
Generally, a transmitter in a MIMO system may have a plurality of antennas, designated mathematically by Nt and a plurality of receivers, U. The receivers U may each have a single antenna or multiple antennas for receipt of data over the communications channel. The transmitter, which may be implemented as a base station, may be configured to simultaneously transmit one symbol for each of the receivers.
The communications channel, however, may introduce a variety of non-idealities to a transmitted signal, such as may be caused by multipath interference, reflections, motion of one or more receivers, or other properties of a communications channel. A vector of transmit messages to be transmitted over the communications channel may be represented by a vector s=[s1 . . . sU]T, where T denotes a transposition operation. The transmitter may generate precoded data symbols, mathematically denoted x, from the transmit messages s based on channel state information corresponding to the communications channel.
In a communications channel that may generally be considered a slowly varying frequency flat fading channel, a vector of signals y received by the U antennas may then be expressed as y=Hx+n. Where H is a matrix corresponding to the communications channel and n is a noise vector. The channel matrix generally refers to a matrix of data which may represent the operation of the communications channel on the transmitted data symbols, including representations of such effects as reflections. Information about the contents of the matrix H, including, for example the matrix itself or the covariance of the matrix, may be referred to as channel state information. The channel state information may change over time.
Channel state information may generally be generated at a receiver based on data symbols received over the communications channel. The channel state information may then be fed back to a transmitter to adjust transmission properties based on the channel state.